Most asymmetrical brief overvoltage events outdoors, that is to say from the L conductor or N conductor to the ground potential, have a fairly high source impedance. The latter is usually between 50Ω and 1 kΩ As explained in more detail below, such events may result in the destruction of the LEDs of external luminaires which are equipped with the latter and have a metal housing, for example street lamps. Such undesirable events are caused, for example, by lightning strikes in the immediate surroundings of such external luminaires but also by lightning strikes in the clouds, in which case high voltage at the luminaire then primarily results from capacitive coupling. As a result of such a process, the luminaire housing may be charged up to several 10 kV, whereas the electronics of the luminaire remain at a low potential via their coupling to the L or N conductor.
If no protective measures are taken for external luminaires in this case, a short high-impedance voltage pulse can easily destroy the LEDs (light emitting diodes) of such an external luminaire since the voltages occurring in this case are very high. These voltages are between 40 kV and 6 kV starting from the strike point to a radius of 300 m around the strike point. These voltages also occur in that form in the LED module of an external luminaire equipped with the latter. A discharge resistance of a few MΩ, for example, which is provided for safety between the housing and ground potential does not suffice to protect against such events.
A conventional overvoltage protection apparatus 10 (SPD—Surge Protection Device) is illustrated in FIG. 1. This has proved successful in street luminaires having a tested PE (Protection Earth) connection. In this case, “protection class I” means that the metal luminaire housing 12 is connected to a tested PE. This overvoltage protection apparatus 10 uses the series circuit including a first and a second varistor V1, V2 between the N conductor and the L conductor, the coupling point of the two varistors V1, V2 being connected to the luminaire housing 12 and therefore to the PE via a spark gap FS1. The two varistors are usually dimensioned in such a manner that they become conductive at 500 V. The spark gap FS1 has a breakdown voltage of between 500 V and 5 kV, the operating voltage being approximately 40 V after a breakdown.
Reliable or tested PEs are unfortunately not always available for external or street lighting, e.g. are available, above all, only in cities. Therefore, a reliable or tested PE, which would be sufficient for protection class I, often does not exist for conventional street lighting.
Therefore, luminaires which have a protection concept according to protection class II are normally used in external or street lighting.
In this case, the overvoltage protection apparatus 10 illustrated in FIG. 1 is not permissible since the metal luminaire housing would have to be connected to the connection point PE here in order to achieve overvoltage protection which is not connected to a reliable PE, however.
Therefore, luminaires in which the luminaire housing is reliably connected to PE are luminaires of protection class I.
Electrical devices, and therefore also luminaires, of protection class II can be identified by the fact that they can be operated using a standardized two-pin Euro plug, that is to say without a PE connection. Such devices have a plastic housing, in which case double/reinforced insulation must be ensured between the mains potential and the housing. The test voltage of such devices is 3 kV AC. The air gaps must be ≧3 mm and the leakage paths must likewise be ≧3 mm.
If discharge lamps are used in external luminaires of protection class I, as previously conventional, the discharge lamps can be operated without a heat sink in the lamps. They are mounted only with the greatest possible distances from the lamp housing since convection is sufficient for heat dissipation in discharge lamps. This is because discharge lamps still operate without major impairment even at a temperature of 300° C.
However, if such external luminaires are equipped with LEDs, it should be taken into account that the efficiency of the LEDs is almost halved, and is considerably reduced in any case, given a temperature increase from 30° C. to 60° C., for example. For this reason, LEDs are usually mounted on a heat sink which is in turn coupled to the luminaire housing for heat dissipation. In order to avoid impairing the heat dissipation too much, a thin insulating layer is provided if necessary between the circuit board material of the LED and the heat sink, for example. However, the distances resulting in this case are not large enough to prevent a flashover of the above-mentioned high voltages occurring as a result of lightning strikes on the lamp housing if it is taken into account that the electronics of the luminaire are still at N potential. A lightning strike in the immediate surroundings of the external luminaire therefore often results in the destruction of the lamp electronics, e.g. the LEDs.
Overvoltage protection would also be desirable for other devices of protection class II, for example devices for controlling traffic flow or other devices for external applications.